(a) Technical Field
The present invention relates to a method for preparing a flexible polylactide stereocomplex and a polylactide stereocomplex prepared thereby.
(b) Background Art
Recently, with the rising concern about the environmental pollution caused by non-degradable, petroleum-based plastics and the depletion of petroleum resources, renewable natural resources such as starch, pectin, protein, etc., are attaining much attention for their applications in food packaging requiring biodegradability and water solubility. Biodegradable polymer materials are valued highly in many fields including medical science, agriculture, ecology, etc. due to their intrinsic degradability properties. More recently, their value is increasing rapidly in the field of ecology and medical science. The polymers can be largely classified into natural biodegradable polymers and synthetic biodegradable polymers. Although natural biodegradable polymers are known as promising due to their good environment-friendliness, physical performance and biocompatibility, they are expensive and it is difficult to control their properties as desired. On the other hand, the synthetic biodegradable polymers are valued highly in that their properties can be fine-tuned as desired. Therefore, their commercial value is highly esteemed these days.
Among the synthetic biodegradable polymer materials, polylactide (PLA) is widely used in the field of ecology and medical science due to its relatively superior performance as well as affinity for the environment and organisms, nontoxicity, or the like. It is used as material in the field of environment such as disposable packaging films, films for agricultural or industrial applications, food packaging containers, or the like. Also, it is developed and used for medical purposes, such as drug delivery systems (DDS), pins and screws for fixing bone and tissue, sutures, or the like. Furthermore, studies are carried out to use them in automobile parts and industrial materials by enhancing the thermal and mechanical stability of the biodegradable polymers.
The development of such new materials is directed to the development of environment-friendly products as well as the improvement of qualities. Accordingly, the need for new materials capable of satisfying such requirements is increasing in the industry. For example, a polymer stereocomplex prepared by melting two single-phase polymers having different enantiomerism or mixing them in an organic solvent at elevated temperature, provides a new crystal structure and better thermal and mechanical stability than that of the single-phase polymers. It may be the new material capable of satisfying the industrial needs [Ikeda et al., Macromolecules, 20, 904 (1987)]. In particular, since the stereocomplex can remarkably improve physical properties and performance of products and extend their service life, it can contribute to the reduction of environmental pollution. The stereocomplex is applicable in various fields depending on the kind and molecular weight of the polymers used, including automobile, packing materials, semiconductors, as well as food, medicine, communications and the military.
When high-molecular-weight linear polylactide is used to prepare the polylactide stereocomplex, the stereocomplex typically results in poor flexibility of the polymer chains because the single-phase polylactide itself has a rigid crystal structure. Also, in this case, it is reported that the formation of the stereocomplex decreases considerably after thermal processing and, as a result, the single-phase polylactide exists together with the polylactide stereocomplex (Penczek et. Al., Macromolecules 2006, 39, 3711). Thus, research has been directed to improve the flexibility of the single-phase polylactide when preparing the polylactide stereocomplex. The Penczek group reported that they prepared a nonlinear, star-shaped polylactide which has a molecular weight similar to that of linear polylactide so as to provide flexibility to the polymer chains, and prepared a polylactide stereocomplex using the polymer. After evaluation of thermal stability, they reported that the degree of stereocomplex formation does not decrease after thermal processing as compared to the linear polylactide.
In general, when preparing the stereocomplex, an organic solvent is used, or direct melt mixing or bulk polymerization is employed without using an organic solvent. Solution casting is used frequently, which requires an organic solvent capable of dissolving biopolymer well to prepare a biodegradable polymer stereocomplex. However, it is difficult to select a suitable organic solvent and a long times are required to completely remove the organic solvent after the stereocomplex is prepared [Tsugi et al., Macromol. Biosci., 5, 569 (2005)]. The melt mixing method requires a high temperature of 200° C. or above, which may accelerate the degradation of the biodegradable polymer. It is known that, in that case, crystallization of the single-phase polymer is likely to be induced rather than the formation of the stereocomplex [Tsugi et al., Macromolecules, 25, 4144 (1992)]. Further, it is reported that the method is limited in preparing a high-molecular-weight, high-strength biodegradable polymer stereocomplex since it is known that the biodegradable polymer that can be prepared by the method has an average weight-average molecular weight of a hundred thousand [Fukushima et al., Macromol. Symp., 224, 133 (2005)]. Thus, it can be seen that the above method is limited in preparing a high-strength biodegradable polymer stereocomplex having good thermal and mechanical stability. For these reasons, researches are ongoing on a new method for preparing a high-strength biodegradable polymer stereocomplex with a large weight-average molecular weight.
Carbon dioxide is a supercritical fluid widely used for its low critical temperature and pressure, low price, incombustibility and nontoxicity. However, the supercritical carbon dioxide has the problem that it cannot dissolve polymers other than fluoropolymers and silicon-based polymers (siloxane polymers).